Today, dynamic optical networks use all optical switching technology such as Wavelength Selective Switch (WSS) embedded in (Remotely) Reconfigurable Add Drop Multiplexer (ROADM). The Generalised Multiprotocol Label Switched (GMPLS) application to photonic networks is called Wavelength Switched Optical Network (WSON).
The International Telecommunication Union ITU-T G.692 Recommendation defines an Optical Supervisory Channel (OSC) as “A channel that is accessed at each optical line amplifier site that is used for maintenance purposes including (but not limited to) remote site alarm reporting, communication necessary for fault location, and orderwire. The Optical Supervisory Channel is not used to carry payload traffic”.
In practical implementations the OSC is an additional wavelength λOSC, usually outside the optical amplifier (Erbium-Doped Fibre Amplifier (EDFA)) amplification band at 1510 nm, 1620 nm, 1310 nm or another proprietary wavelength. The OSC carries information about the DWDM optical signal as well as remote conditions at the optical terminal or amplifier site. It is also normally used for remote software upgrades and network management information. The OSC signal structure is vendor specific, even if the ITU standard suggests using an OC-3 signal structure. The OSC is always terminated at intermediate nodes, where it receives local information before retransmission. This contrasts with wavelengths which carry client signals (i.e. traffic), which are only terminated at endpoints of a lightpath.
Increasing bit rates from 2.5 Gbit/s to 100 Gbit/s and higher, combined with the increasing number of wavelengths from 16 to 160 and the narrowing of the channel spacing, impacts the routing of a lightpath due to physical constraints, which are often referred to as impairments. The effect of such impairments should be considered during the process of computing a lightpath between a source node and a destination node to be sure that the optical signal carried over the lightpath will have sufficient quality to detect the carried traffic at the receiver interface. This quality is usually quantified using a Quality of Transmission (QoT) parameter, like the Q factor, which is strictly related to the Bit Error Rate (BER).
In an opaque optical network, a traffic-carrying optical signal is converted between the optical domain and the electrical domain at each intermediate node. In a transparent optical network a lightpath remains in the optical domain between the source node and destination node, without conversion between the optical domain and electrical domain at intermediate nodes. So, it is only possible to monitor the signal quality at the destination endpoint of a lightpath, such as by measuring the BER at the receiver, or at an intermediate regeneration point, if one is provided along the lightpath. BER provides a measurement of the QoT as resulting from the concurrent action of multiple physical effects without distinguishing among each single cause of detrimental effects.
This scarce and limited availability of quality related information makes it difficult to perform a path computation which is fully aware of the real impact of the impairments that the signal will experience on the path from the source node to the destination node.